As the use of optics has become more prevalent in communications systems, a variety of components used to direct and control light waves have been developed. One such component is an optical attenuator. Optical attenuators are typically used to limit the power of an optical light source. This is for example necessary to limit transmitted power to a receiver.
Often attenuators are combined with other components. For example, an optical modulator is often combined and even integrated with an attenuator. The attenuator reduces the power of an optical light source, so that a modulated light source is provided at a desired output power level.
One known attenuator relies on electro-absorption and is formed as a reverse-biased junction in a semiconductor material. The degree of attenuation is controlled by varying the reverse bias voltage across the junction, the designed optical mode overlap with this junction, and by the separation between the semiconductor material's absorption band-edge and the operating wavelength of the attenuator. Such electro-absorption attenuators may be integrated with a semiconductor optical modulator. Conveniently, electro-absorption attenuators are easily fabricated, take limited space on an integrated circuit, and may be formed in the same grown structure as the optical modulator.
Disadvantageously, for these attenuators the level of attenuation is wavelength, polarization and temperature dependent.
Moreover, when an electro-absorption attenuator is formed as part of an integrated circuit, sharing a common growth structure with other semi-conductor devices, performance of the attenuators and/or the other devices must sometimes be compromised. For example, when an electro-absorption attenuator is integrated with a Mach-Zehnder modulator, the performance of both the attenuator and the modulator must typically be compromised to allow the operation of both devices, since the ideal operating wavelength relative to the semi-conductor band-edge is different for both the attenuator and modulator.
Integrating components creates additional difficulties. For example, multiple electro-absorption attenuators are often placed in series to obtain a desired level of attenuation. This in turn requires increased space on an integrated circuit on a semi-conductor substrate. Further, electro-absorption attenuators must dissipate energy taken from the input signal in some form. Typical, this energy is dissipated as heat, or otherwise absorbed by the substrate on which the attenuator is formed. This energy may impact the performance of proximate components on the substrate.
Another form of attenuator uses a Mach-Zehnder interferometer. Phase modulation in one or two legs of a Mach-Zehnder interferometer is converted to amplitude modulation. The phase modulation may be accomplished by changing the index of refraction of one of the legs. Although this form of attenuator does not suffer from the same problems as conventional electro-absorption attenuators, it does require a greater surface area, which is not desirable in integrated semiconductor components. Historically when this form of attenuator has been formed on an integrated circuit with a Mach-Zehnder modulator it has typically used the same index change mechanism as the modulator.
Clearly then, there is a need for an improved optical attenuator that lends itself to integration with other optical components.